Handle ThetaGPU in GNU make system (#3469)

ThetaGPU gets incorrectly set to use the configuration for Theta, which
does not work because the cray compiler wrappers are not available on
ThetaGPU. However, Make.unknown works fine. So this PR reverts to using
that on ThetaGPU.

The proposed changes:
- [x] fix a bug or incorrect behavior in AMReX
- [ ] add new capabilities to AMReX
- [ ] changes answers in the test suite to more than roundoff level
- [ ] are likely to significantly affect the results of downstream AMReX
users
- [ ] include documentation in the code and/or rst files, if appropriate

Tools/GNUMake/Make.machines

@@ -76,9 +76,11 @@ endif
 
 ifeq ($(findstring alcf.anl.gov, $(host_name)),alcf.anl.gov)
   ifeq ($(findstring theta, $(host_name)), theta)
+  ifneq ($(findstring thetagpu, $(host_name)), thetagpu)
     which_site := alcf
     which_computer := theta
   endif
+  endif
   ifeq ($(findstring polaris, $(host_name)), polaris)
     which_site := alcf
     which_computer := polaris